Solar Gain

Most people on the planet live North of the equator. This means they will experience summer when the Sun is North of the equator, and Winter when the Sun is South of the equator. The Sun appears to cross the equator twice a year: at the Spring and Fall equinoxes; during the year the Sun will travel from its apparent far-northerly position to its apparent far-southerly position, reaching those points at the summer and winter solstices. This web page discusses those two solstices, the angle of the Sun's rays as they strike Earth, and how to align one's walls and windows to make the most of the Sun's position. My almanac shows that the Sun's apparent declination has the maximim North value of 23.44 degrees; it's maximum South value is also 23.44 degrees. This means that while the Sun appears to move from East to West during the day, it also appears to creep North or South throughout the year. (This is because Earth is tilted on its axis by 23.44 degrees in relation to Celestial North.) The first image shows a diagram of this apparent declination.
	This image shows that Earth's axis (which runs from the North Pole to the South Pole) is tilted 23.44 degrees in relation to celestial North. This image shows the Sun appearing South of Earth's equator, which means it is Winter north of Earth's equator, and Summer south of Earth's equator.
	Here we see Earth on the other side of the solar system, six months later. The Sun now appears to be North of the equator; this means it is Summer in the Northern hemisphere, and Winter in the Southern hemisphere. Note that Earth's North Pole is still pointing to the same spot in the sky: the Sun did not actually travel North or South--- it just appears that way to people on Earth who are watching the Sun.
Suppose you live on Earth somewhere around Los Angeles, California, USA. That means your geographic position in relation to the Earth's equator is around 35 degrees North. If you live in Sydney Australia, your geographic position in relation to the equator is around 33.8 South. If you live in Darwin Australia, your're 12.45 South. These figures are your geographic LATITUDE. Here is the main point in all this: since the Sun's apparent movement North and South is at maximum 23.44 degrees, the Sun will NEVER be directly over Los Angeles: the Sun will always appear to be to the South. This means that when the Sun rises in the morning, it will do so just South of East when one is watching in Los Angeles. It will set just South of West. If you live in Sydney, the Sun will ALWAYS appear North of you. You're at 33 degrees South, and the Sun is at maximum 23 degrees South, so at its closest it will appear 10 degrees North of you. If you live in Darwin Australia, which is at 12 degrees South, the Sun can be (and will be for a few hours, twice a year) directly over your head; it will appear to be North of you most of the time, and for a few weeks it will appear South of you. How does this apply to building adobe walls? This knowledge allows you to know how much direct sunlight will strike your walls and windows, and to plan accordingly. If you live around Los Angeles and you want as much Winter sunlight entering your livingroom as possible, you will want to place your livingroom at the South side of the house, and place enough windows on that South-facing wall to heat your livingroom. The point is this: the closer the Sun's apparent latitude is to your own latitude, the sharper the Sun's rays striking you (and your walls), and the hotter it will be. When the Sun is directly over your head, such as can happen in Darwin Australia, it will hit you at 90 degrees at high noon. If you live in Los Angeles, the Sun's rays will hit you (at the Summer Solstice) at an angle of about 78 degrees (your latitude minus the Sun's latitude, subtracted from 90). This also means it will generally be cooler in Los Angeles than Darwin, since the Sun's rays will never strike Los Angeles at 90 degrees, but they will in Darwin. If you look at the image on the right, you will see a thick black line running through the center of the graph. This demarks the Earth's equator. The thick pinkish line demarks the geographic latitude of 35 degrees North (around Los Angeles California USA). The thick blue line shows the Sun's apparent Southern and Northern movement throughout one year--- it goes from 23.44 degrees South, to 23.44 degrees North, and back again. At no time will it ever reach the 35 degree North line and therefore appear directly over Los Angeles---- if you live in Los Angeles, the Sun will always appear to be South of you (and to the East or West depending upon the time of day). If you wish to see what I mean by the Sun always being South if you are in Los Angeles, consider this image. Suppose you have built a house at 35 degrees North latitude; during the day the Sun will appear to rise in the East, pass over-head at noon, and then appear to set in the West--- that is the Sun's diurnal cycle. The Sun also has a seasonal cycle, where the Sun appears to travel Northward and Southward, taking one year to make the trip from all the way North to all the way North again. (It therefore takes half a year to go from all the way 23.44 North to 23.44 South.) Since this house is at 35 North latitude, the Sun will never be directly over-head; it will always appear South. The closer the Sun's latitude gets to your latitude, the hotter it will be because the Sun's rays are at a sharper angle: this causes Summer. In this image, the farther South the Sun is, the colder it gets; at its farthest South, it is Winter. One may examine the same process with a house located South of the equator, in Sydney Australia. Here Summer and Winter have switched locations; this means that while it is Summer in Australia, it is Winter in North America. East and West are, of course, the same, since the Sun still rises in the East and sets in the West (let's hope!). Since Sydney's latitude is 33.8 degrees South, the Sun will never be directly over-head in Sydney.

This image shows a wall located North of the equator, facing South, in Winter. The rays of the sun are striking the wall at a angle of about 55 degrees. The sharper the Sun's rays are hitting an object, the hotter that object will get. This is why Winter is cooler than Summer!

If you are building a house North of the equator, you might wish to place the rooms you wish the warmest in the southern part of the building, and place lots of glass windows in the South wall. For cooler rooms, you will wish to place in the northern part of the house, and have fewer windows.

This is also true of the diurnal cycle: if you wish a room to be warmer in the morning and cooler in the evening, place it in the East part of the house.

My personal preference is to have a bedroom at the North and East walls of a house. This means it will be cooler at night, and it will warm up in the morning (as the Sun rises) sooner than a bedroom loacted at the North and West walls. Windows are placed facing South for solar gain in the Winter. (For Summer, read about awnings over windows below.) Here I have also placed windows on the East and West walls, but none on the North side; this is to keep the North side of the building cooler (this is just an exageration to make the point--- I doubt I'd build a house with no windows in one wall, let alone no doors!).

Here we see the same wall, in the Summertime. The sun has moved farther North, which makes the Sun's rays strike the wall at a sharper angle--- the sharper the ray's angle, the warmer the object gets.

Since the Sun's rays are at a sharper angle in the Summer, they will not penetrate as far through a window (and into a room) as they will when in the Winter. We are talking direct sunlight, not indirect: light will bounce around every which way after it has struck an object. Light is at its most energetic (one can say "hottest") when it has arrived directly from the Sun and not from first having bounced off an object.

Suppose there is a window in this wall. To block that direct Summer sunlight, you could put an awning over the window.

This shows how an awning can be used to block that direct sunlight. The awning length is short enough so that during the Summer, none of the direct sunlight passes through the window. In the Winter, when the Sun's rays are at a more obtuse angle, the direct sunlight may slant under the awning and pass through the window.